Vehicle driveline systems may be operable in two-wheel drive (2WD) modes and four-wheel drive (4WD) modes.
In 2WD modes, torque output from a prime mover is provided to only the front wheels and not the rear wheels (front wheel drive (FWD)) or provided to only the rear wheels and not the front wheels (rear wheel drive (RWD)). In FWD mode the transmission of torque output from the prime mover to the rear wheels is prevented and in RWD mode the transmission of torque output from the prime mover to the front wheels is prevented. In 4WD modes, all four wheels (e.g., front wheels and rear wheels) are operatively coupled to the prime mover such that torque output from the prime mover is transmitted to all four wheels by means of a front driveline and a rear driveline.
2WD modes provide greater fuel economy than 4WD modes and 4WD modes provide greater vehicle stability than 2WD modes. Therefore it is desirable to operate a driveline system in a 2WD mode during on-road driving in normal conditions but switch to operation in 4WD when conditions are such that 4WD is beneficial (e.g., automatically when wheel slip is detected due, for example, to icy roads or off-road terrain such as mud or sand). Vehicle driveline systems are able to switch between two-wheel drive (2WD) modes and four-wheel drive (4WD) modes automatically and/or manually while the vehicle is moving.
In order to switch the operation of driveline systems from 2WD modes to 4WD modes while the vehicle is moving it is necessary to synchronise the front driveline (in FWD mode) or the rear driveline (in RWD mode) with the front wheels or rear wheels respectively while the vehicle is moving. Once the speed of the driveline matches the wheel speed a disconnect device can be controlled to enable transmission of torque output from the prime mover via the driveline to the wheels.
It is an aim of the present invention to improve the aforementioned switching of a driveline system.